Vacuum switch having an improved electrode tip



Oct. 25, 1966' F. WATERTON 3,281,563

VACUUM SWITCH HAVING AN IMPROVED ELECTRODE TIP Filed March 16, 1964 United States Patent 3,281,563 VACUUM SWITCH HAVING AN IMPROVED ELECTRODE TIP Frank Waterton, Potters Bar, Middlesex,

or to Associated Electrical Industries England, a British company Filed Mar. 16, 1964, Ser. No. 352,080 Claims priority, application Great Britain, Apr. 23, 1963, 15,995/ 63 6 Claims. (Cl. 200-466) The present invention relates to vacuum switch contacts of the type comprising a metal body with a tip having a multi-metallic free surface.

The surface of the tip is composite and is formed partly from a first metal which has a high arcing voltage and partly from a second metal which has a low arcing voltage. With such a tip the arc burns and is interrupted on the low arcing voltage metal 'whereas the main contact is made with the high arcingvoltage metal and is substantially free from burning. The low arcing voltage metal may be contained in reservoirs formed by recesses in the free surface of the mass of high arcing voltage metal and the low arcing voltage metal will spread over the free surface of the mass of high arcing voltage metal during operation of the contact.

The life of a contact of this type is limited by the materials forming the surface of the contact continuing to perform their required functions without major erosion. The low arcing voltage metal evaporates when the arc is formed and most of the vapour returns to the contact surfaces. However, some of the vapour escapes and the mass of low arcing voltage metal is gradually reduced. As the mass of low arcing voltage metal is reduced to near zero more of the surface of the mass of high arcing voltage metal is exposed and the wear of this metal starts.

It is found that with known contacts of the above type, using for example molybdenum or steel for the high arcing voltage, metal with a nickel/ tin/ copper alloy for the low arcing voltage metal, the rate of wear of the high arcing voltage metal increases rapidly when as little as 10% of the low arcing voltage metal has evaporated. It is also found that the rate of wear is dependent on the average current density over the contact and the rate of wear/current density curve shows a very sharp rise at a particular value of the current density, depending on the materials of the contact. This additional discovery indicates that the rate of wear is dependent not only on the evaporation of the low arcing voltage metal, but on other features as well.

Metallurgical examination of worn contacts has indicated that two main causes are responsible for the premature breakdown of the contacts. These are:

(1) Grain boundary penetration of the high arcing voltage metal by the low arcing voltage metal resulting from mechanical and thermal fatigue cracking of the crystals and crystal boundaries in the high arcing voltage metal at the surface.

(2) Cracking in the crystals (below the surface) of the high arcing voltage metal due to thermal fatigue.

These effects are dependent on'the working temperature of the contact i.e. on the rate of operation of the switch and the size and thermal conductivity of the contacts and their supports, and also on the recrystallisation temperature of the high arcing voltage metal of the contacts and on the average current density on the contacts.

These phenomena indicate that in order to achieve maximum contact life the high arcing voltage metal must be carefully chosen so that it has a close grain structure and high resistance to thermal fatigue in the grains themselves and as high a recrystallisation temperature and resistance to recrystallisation as possible.

England, assign- Limited, London,

3,281,563 Patented Oct. 25, 1966 The object of the present invention is to provide an im proved vacuum switch having a contact of the above type in which the rate of wear of the high arcing voltage metal is reduced.

According to the present invention a vacuum switch incorporates a contact which comprises a tip having a multimetallic contacting surface formed partly from a high arcing voltage alloy including titanium and partly from a low arcing voltage metal.

Preferably the high arcing voltage alloy comprises molybdenum with /2 of titanium by weight.

By including a small percentage of titanium in the alloy forming the high arcing voltage metal it is found that the life of a contact is increased by a factor of about four times due to a decrease in the rate of wear of the high arcing voltage metal. For example an average wear rate of 0.001 in. per 7,500 operations has been reduced to 0.001 in. per 30,000 operations at the same current density. This corresponds in certain arrangements to an extension of the useful life of the contact from 200,000 to 800,000 operations.

With such a decrease in the rate of wear of the contact surface the life of the contact tends to be limited by the evaporation of the low arcing voltage metal and not by the erosion of the high arcing voltage metal, and hence the life of the contact is increased.

A vacuum switch in accordance with the invention is illustrated in the accompanying drawing in which:

FIG. 1 is a side view sectioned on an axial plane of an electrical vacuum switch, and

FIG. 2 is a plan view of one of the contacts of the switch.

FIG. 3 is a plan view of a modified contact.

With reference to the figures the vacuum switch comprises a chamber having a body 1 of electrically insulating material and two end plates 2, 3 of electrically conducting material. An aperture 4 is formed in one end plate 2 and a movable conductor 5 extends through this aperture. A bellows member 6 extends between conductor 5 and the end plate 2 so as to seal the aperture 4 but to permit movement of the conductor 5 relative to the end plate 2.

A fixed conductor 7 extends through the other end plate 3. The inner ends of the conductors are adjacent and are formed respectively with contact-s 8, 9 at least one of which is formed with a tip constructed in accordance with the invention and comprising a high arcing voltage metal and a low arcing voltage metal.

The contacting surface of the tip of contact 8 is illustrated in FIG. 2 as being formed with a plurality of circular recesses 11. The main portion 12 of the tip is formed from a high arcing voltage alloy including of titanium by weight and molybdenum or steel, while the recesses 11 are filled with a low arcing voltage metal, for example an alloy of nickel/tin/copper. The low arcing voltage material could alternatively be contained in recesses of other shapes, for example one or more annular recesses generally as shown in FIG. 3. The recesses need not be located symmetrically in the contacting surface of the tip.

A gas at a very low pressure of the order of 10- mm. Hg or less is contained within the chamber and a current flowing in the conductors 5, 7 when the contacting surfaces of the tips of the contacts abut is arranged to be interrupted by moving conductor 5 so that the tips of the contacts separate. One or more arcs will then be formed between the free contacting surfaces of the tips of the contacts and, as described above, the ends of these arcs burn on the low arcing voltage in the recesses 11 of each contact tip. The mass of low arcing voltage metal tends to be reduce-d with successive operations of the switch, and, as described above, this results in the wear of the high arcing voltage metal forming the portion 12 of each contact.

By using as the high arcing voltage metal an alloy contai-ning titanium in accordance With the invention this we at is appreciably reduced.

What I claim is:

1. A vacuum switch comprising:

(a) an evacuated envelope,

(b two relatively movable electrodes extending into the envelope,

(c) a tip provided at the inner end of an electrode within the envelope and having a contact surface arranged to engage with the co-operating contact surface of the other electrode,

(d) a multiplicity of recesses provided in the contact surface of the tip and containing a metal of low arcing voltage,

(e) the tip consisting essentially of molybdenum alloyed with titanium effective to increase the temperature of recrystallization of the tip, whereby in use of the switch the rate of electrical Wear of the tip produced by such recrystallization as a result of disengagement of the electrodes to interrupt a current is decreased.

2. A vacuum switch according to claim 1, wherein the molybdenum of the tip is alloyed with 0.5% by weight of titanium.

3'. A vacuum switch according to claim 1, wherein the low arcing voltage metal is an all-0y of nickel with tin and copper.

4. A vacuum switch according to claim 1, wherein the recesses are in the form of blind holes of substantially circular cross-section.

5. A vacuum switch according to claim 1, wherein the recesses are in the form of annular channels.

6. A vacuum switch according to claim 1, wherein the tip is provided on both electrodes.

References Cited by the Examiner UNITED STATES PATENTS 1,63 3,258 6/1927 Laise 200-166 2,179,960 11/1939 Schwarzkopf 200-l6 6 X 2,362,007 ll/ 1944 Heusel et al 200-166 X 2,900,476 8/1959 Reece 200-166 X 3,014,110 12/1961 Cobine 200-144 ROBERT K. SCHAEFER, Primary Examiner.

KATHLEEN H. CLAFFY, Exwmin'er.

H. O. JONES, Assistant Examiner. 

1. A VACUUM SWITCH COMPRISING: (A) AN EVACUATED ENVELOPE, (B) TWO RELATIVELY MOVABLE ELECTRODES EXTENDING INTO THE ENVELOPE, (C) A TIP PROVIDED AT THE INNER END OF AN ELECTRODE WITHIN THE ENVELOPE AND HAVING A CONTACT SURFACE ARRANGED TO ENGAGE WITH THE CO-OPERATING CONTACT SURFACE OF THE OTHER ELECTRODE, (D) A MULTIPLICITY OF RECESSES PROVIDED IN THE CONTACT SURFACE OF THE TIP AND CONTAINING A METAL OF LOW ARCING VOLTAGE, (E) THE TIP CONSISTING ESSENTIALLY OF MOLYBDENUM ALLOYED WITH TITANIUM EFFECTIVE TO INCREASE THE TEMPERATURE OF RECRYSTALLIZATION OF THE TIP, WHEREBY IN USE OF THE SWITCH THE RATE OF ELECTRICAL WEAR OF THE TIP PRODUCED BY SUCH RECRYSTALLIZATION AS A RESULT OF DISENGAGEMENT OF THE ELECTRODES TO INTERRUPT A CURRENT IS DECREASED. 